In recent years, optical recording media have been put to wide and practical use in the forms of, for example, a CD and a DVD, as media for storing music and images. For further expansion of the field of application, such optical media have been undergoing improvements in capacity and performance.
Hereinafter, an example of a conventional optical recording medium will be described with reference to appended drawings.
FIG. 23 is a schematic diagram showing an example of a conventional optical recording medium. In FIG. 23, servo areas 1403 and data areas 1404 are formed along a spiral track center line 1402 (only a part thereof is shown in FIG. 23) on a conventional optical recording medium 1401.
FIG. 24 shows the respective configurations of the servo area 1403 and the data area 1404. As shown in FIG. 24, a clock mark 1501, a first wobble mark 1502 and a second wobble mark 1503 are formed at a predetermined distance from each other in the servo area 1403.
Furthermore, in the data area 1404, information is recorded substantially on the track center line 1402. The clock mark 1501 is used for generating a synchronizing clock signal used to reproduce the first wobble mark 1502, the second wobble mark 1503 and information recorded in the data areas 1404.
The track center line 1402 is a virtual line obtained by sequentially connecting the respective centers of the clock marks 1501.
The first wobble mark 1502 and the second wobble mark 1503 are formed respectively in positions shifted in opposite directions by a predetermined distance in the radial direction with respect to the track center line 1402, and are used to detect a tracking error signal corresponding to a positional shift of a beam spot irradiated onto the optical recording medium 1401 from the track center line 1402.
When the beam spot is shifted from the track center line 1402, one of the light beams reflected from the first wobble mark 1502 and the second wobble mark 1503 becomes larger than the other reflected light beam, and thus a tracking error signal can be detected.
In the optical recording medium 1401 with the above-described configuration, a tracking error signal is detected with reference to the synchronizing clock signal generated based on the clock mark 1501.
With the use of the tracking error signal, tracking control is performed so that the beam spot on the optical recording medium 1401 scans the information recorded on the date area 1404 accurately, and thus information recording or reproduction can be performed.
However, in the above-described configuration, a tracking error signal is obtained in a discrete manner, and thus the following problem arises. That is, when an eccentricity amount representing a relative displacement amount in a radial direction between an optical recording medium and a beam spot irradiated onto the optical recording medium is small, the eccentricity amount can be measured based on the tracking error signal; however, when the eccentricity amount is large, a tracking signal cannot be detected accurately, and thus the eccentricity amount cannot be measured.
As a result, an eccentricity amount correction operation cannot be performed, thereby making it difficult to perform a pull-in operation of tracking control or the like, which has been disadvantageous.
JP 63(1988)-225924 A discloses an optical disk as a measure to solve the above-mentioned problem. That is, “the optical disk is characterized in that a distance between at least one pit of a pair of wobble pits and a clock pit as a reference pit to information data varies between at least three predetermined distances, and the distance is changed in a predetermined order repeatedly for every at least one information track”, thereby allowing the above-mentioned problem to be solved.
According to this configuration, an eccentricity amount representing a relative displacement amount in a radial direction between an optical recording medium and a beam spot irradiated onto the optical recording medium can be detected.
Thus, an eccentricity amount correction operation can be performed, thereby allowing a stable pull-in operation of tracking control or the like to be realized.
However, it has been revealed that, in the above-mentioned conventional optical recording medium, when detecting a direction of a relative movement in the radial direction between the optical recording medium and the beam spot irradiated onto the optical recording medium, a detection error may occur depending on how the wobble marks are arranged. This has led to the problem of the deterioration in accessing accuracy.